Exchanges of data between a data-processing unit and working stations generally take place via cable connections. In a known method of connection, termed star connection, each of the items of apparatus at a working station is connected to the unit by a cable associated with it. Such a connection does away with the necessity for an addressing circuit to be fitted to each item of apparatus and, by the use of standardized plugs and sockets, allows the user to make up the working station to suit his needs. However, star connections make it necessary for any given working station to have as many cables as there are items of apparatus at the station and require wiring done at the time of installation to be such as to indicate to the data-processing unit which socket corresponds to which item of apparatus. Also, since the input/output management capabilities of the data-processing unit are limited, the number of items of apparatus which can be connected must remain limited if the design of the unit is not to become excessively complicated.
The exchange of data between a processing unit and an apparatus is the responsibility of a pair of data transmitting and receiving circuits which are situated at respective ends of the cable connecting the apparatus to the unit. If the number of items of apparatus is high, the number of transmitter/receiver circuits is correspondingly greater. Because the plugs and sockets are standardized at the end of the cable connected to the unit, it is necessary when a working station is brought into operation to indicate to the unit the type of apparatus which is connected to the concerned plug. Type should be understood to cover not only the general type of peripheral at the apparatus (keyboard, display screen, ETC.) but also sub-types within a given type; for example whether a keyboard is digital or alpha-numeric. Each item of apparatus must therefore include a circuit to allow it to recognize a type query from the unit and to reply by transmitting to the unit an indication of its type. While star connections theoretically allow the unit to operate simultaneously with the various different items of apparatus connected to it, this possibility is not usually available because of the sequential nature of addressing software which the unit employs to establish a communication link with the items of apparatus at one and the same working station.
One of the objects of the present invention is to provide an interface for connecting a data-processing unit to a plurality of working stations, which interface has a small number of data transmission channels and transmitter/receiver circuits, thus reducing its manufacturing costs in comparison with known interfaces.
Another object of the invention is to provide apparatus that can be removed from or added to a working station without the necessity of increasing or removing circuits at the inputs and outputs of the unit at the time of installation.
Another object of the invention is to provide a new and improved interface between a data processing unit and a number of working stations, which interface allows a large number of items of apparatus to be provided at the same working station.
Another object of the invention is to provide a new and improved interface between a data processing unit and a number of working stations, wherein connections to a transmission means are more readily established than those used in the prior art, such as the star connection.
Another object of the invention is to provide reliable apparatus for exchanging data between a data-processing unit and items of apparatus at stations which are connected to the unit.